Process and system for producing commercial quality carbon dioxide from high solids lime mud

Abstract

The invention features methods and systems for producing commercial quality carbon dioxide (CO2) of 90% to +99% purity using, wet calcium carbonate lime mud produced in a manufacturing operation, for instance, Kraft pulp mill lime mud (a.k.a., “lime mud”) as a feedstock to a multi-stage lime mud calcination process. This process may be fueled with waste water treatment plant (WWTP), sludge biomass, precipitated lignins, coal, or other low cost solid fuels. High reactivity “soft-burned” lime product (“calcine”) required in the mill's chemical recovery circuit is also produced, and steam and heated boiler feed-water is generated and exported to the mill's steam distribution and generation system as well as hot process water for use in the mill's boiler house and manufacturing operation. The system for calcining calcium carbonate lime mud produced from a re-causticizing manufacturing operation and converting it to re-burned lime and CO2 comprises a calciner and a combustor linked by a moving media heat transfer (MMHT) system or apparatus. The MMHT system or apparatus thermally links separate fluid bed combustion (exothermic) and calcination (endothermic) stages with a solid particulate media. The system further comprises a flash dryer or spray dryer that utilizes exhausted heat from the calcination stage.

Description

FIELD OF THE INVENTION[0001]The present invention provides methods and systems for producing high quality carbon dioxide (CO2). Further, the present invention provides an improved process for calcining or “re-burning” calcium carbonate or “lime mud” as produced in, for instance, a Kraft or other alkali-based paper pulp manufacturing operation. Such lime mud may be converted to commercial grade CO2 and high-quality re-burned lime using only low cost biomass and biomass derived negative cost waste-water treatment plant (WWTP) sludge, non-condensable waste mill gas (NCG), or low cost solid fuels such as coal, petroleum coke, etc. The biomass and biomass related fuels are also “carbon-neutral” with regards to their global environmental impact.BACKGROUND OF INVENTION[0002]The major global consuming regions for cryogenic liquid CO2 are the United States, Western Europe, and Japan. The United States is the largest consumer at about two thirds of the total global amount. Globally, the major...

AI Technical Summary

Benefits of technology

[0039]Additionally, the method may include mixing the wet lime mud feed to the spray dryer or flash dryer with H2O2, to convert sodium sulfide contained in said lime mud into Na2SO4. Aqueous solutions of Na2CO3, or Na2SO4 may also be mixed to change the Na2CO3 / Na2SO4 ratio within the lime mud to a higher melting point. This may be particulary effective to mitigate calciner scaling and fouling and unwanted gaseous emissions. Still further, the method may include injecting the dry lime mud feed to the calciner at the base of the fluidized media bed of the calciner. This may help to maximize dry lime mud particle residence time thereby ensuring thorough calcination.

Problems solved by technology

In the United States, there is increasing CO2 availability from bio-ethanol production due to the growing need for clean transport fuels and chemicals, and decreased availability due to declining ammonia production caused by high natural gas prices.

This has created a supply and demand balancing dilemma for both producers and consumers of liquid CO2

The conventional lime mud calcination process has, however, not easily been converted to biomass fuels and remains a conspicuous consumer of high cost, greenhouse gas emitting fossil fuels.

It fell into disuse, however, as rotary kiln / LMD technology re-captured the fuel economy lead and FluoSolids installations experienced operability issues and an inability to economically operate at the high unit capacities required by a “world-class” Kraft pulp mill.

Low-cost solid fuels such as biomass, waste water treatment plant (WWTP) sludge, coal etc. are typically not used as-is due to their contaminating ash content.

Wet WWTP sludge and biomass have the added penalty of lower adiabatic flame temperature.

Due to technology limitations, attaining future significant fossil fuel consumption / cost reductions in the rotary kiln / LMD calcination process appears difficult.

There is, however, wasted energy within the rotary kiln / LMD calcination process that could be recovered with the proper technical approach.

Such energy efficiency, however, is not possible with the rotary kiln / LMD calcination process since a very large non-variable fuel amount is required to provide the constant endothermic heat-of-reaction enthalpy and also heat reaction products (CaO and CO2) to the calcination temperature.

The following kiln pre-heat section, however, has insufficient chain heat transfer ability to absorb available energy from the combustion products and CO2 associated with the aforementioned non-variable fuel component and transfer it into the dried solids entering from the drying zone.

Unfortunately, the current rotary kiln / LMD technology is limited in the ability to economically respond to this fuel saving opportunity and will become less fuel-efficient as filter cake solids content further increases.

The less utilized fluidized bed calcination process never featured a solids pre-heat section, and wastefully dissipated this excess heat via a water spray cooler to control lime mud flash dryer exit temperature.

Designs have also been proposed to address this dilemma by inserting a waste heat boiler in place of the spray cooler step, but this may never be commercialized due to the high surface fouling characteristics of calciner exit gas caused by the presence of low eutectic melting point Na2CO3 / Na2SO4 mixtures.

Many of these emissions have been reduced or eliminated thanks to better manufacturing practices but WWTP sludge (cellulosic, organic, and inorganic matter from waste water treatment) remains a costly disposal problem since it must ultimately be placed in a landfill.

As previously discussed, WWTP sludge cannot be used in existing rotary kiln representing a lost opportunity to conserve fossil fuels.

While NCG combustion in rotary kilns has been widely practiced, operability problems (kiln deposit “ringing”, SO2 “blow-through”, etc.) persist at most mills Accordingly, stand alone NCG incinerator / boilers that raise steam and scrub sulfurous emissions are increasingly used.

These incinerator / boilers, however, are not always available when NCGs are produced so a back-up disposal means is desirable.

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